One recurring problem which can arise during the operation of dynamic random access memories (DRAMs) is the degradation of data stored on the DRAM cell capacitors due to minority carrier injection into the substrate. This typically occurs when the address and data lines to the memory device switch low and the corresponding bond pads, which reside on the surface of the substrate, spike below zero volts. This spiking can become particularly pronounced if the inductance of the address and data lines is not well controlled.
In an n-channel memory cell storing a logic 1, a depletion area is created around the n+ doped region common to the access transistor and the storage capacitor. Minority carriers, in this case electrons, injected into the substrate from the spiking can recombine with the holes in the depletion area and cause charge to leak-off the storage capacitor. By biasing the substrate at a negative voltage, diodes formed at the bond pad structures cannot be biased by an undershoot and thus, electron injection into the substrate is prevented.
A negative substrate bias is achieved typically by pumping electrons into the substrate with a substrate pump. Generally, pumping is performed until the substrate reaches equilibrium and before any writes are made to the cells of the array (electrons from pad undershoot in contrast are appear quickly and if allowed to reach the array, cause leakage).
This technique has serious drawbacks when a large array of DRAM cells has been fabricated on a large substrate. Among other things, the substrate voltage at different locations around the substrate may differ substantially, depending on the distance from the charge pump, the distance from the bond pads, and the substrate resistance. For example, the substrate voltage in regions significantly separated from the charge pump may not be sufficiently reduced to effectively control data degradation in the cells fabricated those regions. This problem may be solved by using a charge pump with larger capacitors; however, even with larger capacitors some voltage drop off will still occur as the distance from the charge pump increases.
Thus, the need has arisen for circuits, systems and methods for controlling the voltage bias of a substrate. In particular, such circuits, systems and methods should be applicable to control the DRAM cell leakage from charge injection into large substrates and/or substrates supporting large cell arrays.